not sure if it'll work, but did you try booleaning just one text at a time? and while soing so, try to make the text thicker and just insert it like 1/4 into the tombstone.
Otherwise, if it doesn't work, try this technique. put the text letters in place. > go to front view where you can see a view facing it front on. THen use the split polygon tool and trace the otuline of the text onto the tombstone > then use split polygon again to add in lines where there should be lines and > extrude. Hope that helps, although it will take longer to do it this way.
9 months into Maya...and still learning...
0 experience with any other 3D programs